I have a '91 Mustang GT. The motor has headers, underdrive pulleys, and a March air induction. Nothing radical. Lately when I am decellerating, there is a squealing noise that comes from what sounds like one of the pulleys. I'm sure that it is not the serpentine belt, and the tensioner seems to be working fine. It only happens when the car is decellerating, and will not make the noise when the vehicle is sitting still. Help!

 

Silicone gease on the belt may help or you might have a bearing going bad.